1. Field of the Invention
The present invention relates to a sheet processing apparatus that has a function of aligning sheets stacked on a tray, a control method for the sheet processing apparatus, and a storage medium storing a program.
2. Description of the Related Art
For sheet stackers that stack printed sheets discharged from a printing apparatus on a tray, there has been demand for the ability to align the sheets on the tray with a high degree of accuracy. Japanese Patent Laid-Open No. 2006-206331 suggests a technique to align sheets stacked on a stack tray. According to this technique, alignment members are provided on the stack tray, and the positions of edge surfaces of the sheets parallel to a sheet discharge direction are aligned by the alignment members coming into and out of contact with the edge surfaces of the sheets.
Now, assume the case where, as shown in FIG. 20, sheets having a sheet width W2 are to be stacked and aligned on sheets that have a different sheet width W1 and are already stacked on a stack tray 2001 (W2<W1). In this case, in order to align the sheets having the sheet width W2, it is necessary to cause alignment plates A and B to come into contact with a top surface of the already-stacked sheets. When sheet alignment operations are executed by moving the alignment plate A in the direction of an arrow shown in FIG. 20 while the alignment plates A and B are thus in contact with the top surface of the already-stacked sheets, the bottom surface of the alignment plate A is slid against the front surface of the top sheet of the already-stacked sheets. This can lead to the possibility that the toner printed on the topmost sheet of the already-stacked sheets is removed and the quality of an image on the topmost sheet is degraded.
There is also the possibility that the removed toner is attached to and stains the bottom surface of the alignment plate A, and the attached toner stains other sheets when the bottom surface of the alignment plate A comes into contact with other sheets. Upon printing sheets that have a different width from sheets that are already printed and stacked, the printing may be interrupted first to allow a user to remove the sheets that are already printed and stacked from the stack tray, and then the printing may be resumed for the sheets that have a different width. Although this method does not give rise to the aforementioned problem, it is still problematic in that it reduces the productivity of printing.